Is your side project a fork from an open-source project?
I built a Chrome extension around my site (www.zenuml.com). This extension itself is a fork of the famous web-marker (https://github.com/chinchang/web-maker). It has saved a lot of work and I can merge his improvement into my project.
My extension is NOT competing with chinchang's idea at all. ZenUML Web Sequence is to generate sequence diagram and web-maker is a playground for HTML/JS/CSS. From another angle they are similar, because both are from code to DOM.
Are you also forking an open-source project?
I created and still maintain https://github.com/gitpitch/gitpitch. And I forked it as my starting point to ship https://gitpitch.com.
Well done!
I am maintaining https://konvajs.github.io/. It is 2d canvas javascript framework.
The project is a fork of KineticJS - quite popular OSS back in several years ago. But it was closed. So I started to fork. Now I have many freelance leads that come from konva.
Doesn't matter if you compete with the author of web-maker -- he licensed it MIT so you can knock yourself out making commercial software that does compete with his, no problem
I used color-theif to build https://www.hdrainbow.com
Other than having a ad link from that site to my other site(https://www.watermark.ink) I am not directly profiting from that site. It taught me a lot of things from producthunt post to visitor tracking etc.
*Everyones software project is a fork from an open-source project.
*unless you are using some closed source non-sense for some wild reason.